DE-OS 27 05 382 describes a large group of cellulose derivatives, including cellulose acetates. These are to be converted into an anisotropic spinning solution in order to produce filaments by conventional spinning processes. According to Table 1, the degree of substitution (DS) of the cellulose acetate must be between 1.89 and 2.45. As suitable solvents for the solution are indicated predominantly halogenized and phenolic compounds. In connection with the additional cellulose derivatives indicated in DE-OS 27 05 382, also acetone is listed among a large number of solvents. The cellulose derivatives must be present in the spinning solution in an amount of at least 15 vol.%, to meet the requirement of optical anisotropism. However, in DE-OS 27 05 382 the combination "cellulose acetate dissolved in acetone" is not mentioned. Further, a 15-vol, % acetone solution of the cellulose acetate described in DE-OS 27 05 382 would not be optically anisotropic, but would be optically isotropic.
A cellulose acetate of 1.89 degree of substitution (DS) is not soluble in acetone. This is evident from Houben-Weyl "Methoden der orgnischen Chemie" Vol. E 20, Part 3, "Macromolecular Substances", Georg Thieme Verlag Stuttgart, N.Y., 1987, p.2099. According to the DE-OS 27 05 382, cellulose derivatives with a mean degree of polymerization (DP) Of at least 100 anhydroglucose units are said to have a sufficiently high molecular weight to be suitable for the production of filaments. Precise statements concerning the degree of polymerization of cellulose acetate of DS 1.89-2.45 are not stated in DE-OS 27 05 382. This is discussed in Ullmann's Encyclopedia of Industrial Chemistry, 5th fully revised edition, Vol. A5, pp 447-448 in connection with the spinning of "secondary acetate" dissolved in acetone. By this is to be understood "cellulose-2.5-acetate". The number "2.5" means the average number of acetyl groups per anhydroglucose unit. For the production of fibers and cigarette filter tow there is stipulated for the cellulose-2.5-acetate a degree of polymerization (DP) of 300 (cf. page 447, Table 15). It is said to be spinnable from an acetone solution, the viscosity of which at a concentration of 20 to 30% of cellulose-2.5-acetate and at a temperature of 45.degree. to 55.degree. .C is between 300 and 500 Pa.s (probably what is meant is: . . . between 30 and 50 Pa.s . . . ). Filaments obtained therewith have the following physical properties: Tensile strength (cN/dtex) 1.0 to 1.5, elongation (%) 25 to 30, density (g/cm.sup.3) 1.33, melting point (.degree.C.) 225 to 250, and, as already stated, a degree of polymerization of 300.
A summary of the prior art shows that in connection with acetone as solvent of a cellulose-2.5-acetate spinning solution a high degree of polymerization of 300 is stipulated with the result that the spinning solution has a low cellulose-2.5 acetate concentration of about 20 to 30%. With a starting material of this degree of polymerization, its concentration in acetone cannot be raised further for the production of a spinning solution because this would necessarily involve such a great increase of the viscosity of the spinning solution that the latter could no longer be spun with conventional spinning devices (for example at 40 to 50%). A substantially increased concentration would have the advantage that the acetone content in the spinning solution could be drastically reduced with the result of a considerable energy saving in the circulation and recovery of the acetone.